Unusual artefacts in volume render
I had a look at the suggestion from @brockmann. My first attempt was to simply up the samples. The following image is using 2000 samples.



As you can see - all that has really happened is that the artefacts have moved ever so slightly to the centre.

I looked into the intel denoising. This is a feature that has been added in 2.81. I used the following settings (compositing nodes):


Quite simple really. At 500 samples, the following image was the result:



And this is at 2000 samples (same denoising setup):